1. Field of the Invention
This invention relates to a tunable Fabry-Perot Interferometer, which selectively filters a series of monochromatic wavelengths from an incident broadband radiation.
2. Description of the Related Art
Industry experts agree that the telecommunications industry is experiencing explosive growth and is one of today""s fastest growing economic segments. With the tremendous growth of the Internet and the increase in telecommunications traffic, many telecom companies are rapidly deploying new network topologies and transport technologies such as WDM (wavelength-division-multiplexing) and DWDM (dense-wavelength division multiplexing) to increase the capacities of their networks. With the advent of fiber optic communications networks, the deployment of all-optical networks is clearly the ultimate goal for the next generation of telecommunications networks. A critical component to the successful deployment of the all-optical network is a tunable Fabry-Perot Interferometer, which selectively filters a series of monochromatic wavelengths from incident radiation.
As shown in FIG. 1, a Fabry-Perot interferometer 10 is a comparatively simple structure consisting of two plane parallel partial reflectors 12 and 14 (with reflectivity of ca. 90% or higher and with very low losses due to scattering and absorption) separated by a suitable transparent medium. The reflectors may be formed with a concave curvature to minimize losses caused by beam walk off. If the optical path length (distance multiplied by refractive index) between the two reflecting surfaces is an integral number of half wavelengths of the incident light, then the structure becomes optically resonant (i.e. the light at that wavelength is transmitted through the cavity). Other wavelengths not meeting the resonant condition are not transmitted. The wavelengths transmitted by the device are given by                               T          λ                =                                            I                              T                ,                λ                                                    I                              0                ,                λ                                              =                      1                          1              +                              F                ⁢                                  xe2x80x83                                ⁢                                                      sin                    2                                    ⁡                                      (                                                                  2                        ⁢                                                  xe2x80x83                                                ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                        nd                                            λ                                        )                                                                                                          Eqn        .                  xe2x80x83                ⁢        1            
where IT,xcex and Io,xcex are the transmitted and incident light intensities at wavelength xcex, respectively; n is the refractive index of the medium between the two parallel mirrors with separation d. The resonant wavelength can be manipulated (tuned) by changing either the refractive index (n) of the medium between the two mirrors, or the separation between them (d). F is a parameter related to the cavity Finesse, and is related to the mirror reflectivityxcx9chigher F represents narrower transmitted peaks.
The capacity of a WDM or DWDM network is directly proportional to the number of optical signals, or channels, it transports. The initial 400-GHz to 800-GHz 8-channel systems that were deployed in medium and long haul network applications have quickly evolved into conventional C-band 100-GHz systems that incorporate more than 40 channels, but these systems are already beginning to reach their maximum capacities. Many vendors are looking to deploy solutions that will allow a) higher density channel spacing, and b) operations in both the C-band (1530 nm-1563 nm) and the long L-band (1575 nm-1610 nm) simultaneously. These changes will provide the capability to transport up to 256 channels of optical data on a single fiber.
As shown in FIG. 2, a tunable Fabry-Perot interferometer with an initial optical path length of ca. 10 microns is characterized by a filter function 16 with a free spectral range (separation between two transmitted wavelengths) of ca. 125 nm in the C and L-band range. If the optical path length is changed by 1 xcexcm, then the transmitted peaks move horizontally by ca. 100 nmxcx9cthereby scanning the C and the L bands. To provide the desired 256 channels, the filter function must exhibit a Finesse (FSR divided by the full width at half maximum) of at least ca. 200, preferably higher than 2000; and remain undistorted during tuning. As shown in FIG. 2, the transmitted peaks become narrower as the mirror reflectivity (denoted next to the four traces) increases. The two mirrors must have high reflectivity, low loss and low surface defects and they must be aligned to nearly zero tilt between them in the initial state. Further, this alignment state must be maintained as the optical path length between them is changed over the desired range (i.e. by ca. 1 xcexcm). To achieve commercial success, the interferometer must also be cost effective, have low insertion losses, sufficiently rapid scan rates and remain thermally stable over a minimum ten year lifetime.
Currently the majority of commercially available scanning Fabry-Perot interferometers are based on a piezo-electric crystal technology. The movable mirror is mounted on a piezo electric material, which changes its dimensions in response to an applied voltage. Queensgate Instruments Limited produces a piezo controlled interferometer, early versions of which are described by T R Hicks, N K Reay and P D Atherton xe2x80x9cThe application of capacitance micrometry to the control of Fabry-Perot etalonsxe2x80x9d J. Physics E: Sci. Instrum., Vol. 17, 19844 and European patent publication EP702205A2 entitled xe2x80x9cInterferometerxe2x80x9d to T R Hicks. U.S. Pat. Nos. 4,400,058 and 4,553,816 also describe piezo electric interferometers. Piezo interferometers are complex, expensive, operate at high voltages, exhibit limited tuning ranges and have hysterisis and thermal drift problems.
Many attempts have been and are continuing to be made to apply classic silicon micromachining to solve the problems. In the silicon-MEMS based devices, electrodes are deposited onto both mirrorsxcx9cthus an electrostatic force is created when a voltage is applied. The moveable mirror then moves against a semi-rigid silicon micromachined member, thereby changing the mirror separation. When the applied voltage is removed, the restoring force exerted by the semi-rigid member moves the moveable mirror back to its original configuration. U.S. Pat. No. 4,203,128 to Guckel; U.S. Pat. No. 4,825,262 to Mallinson; U.S. Pat. No. 4,859,060 to Katagiri; U.S. Pat. No. 5,561,523 to Blomberg; and U.S. Pat. No. 6,078,395 to: Jourdain;. PCT Application WO99/34484 to Tayebati and E. Ollier; P. Mottier xe2x80x9cMicro-Opto-Electro-Mechanical Systems: Recent developments and LETI""s activitiesxe2x80x9d Proceedings of SPIE Vol. 4075, 2000 each describe variations on a silicon-MEMS based interferometer. These devices require complex processing techniques, which makes them expensive due to the high cost/low yield of precision lithography, exhibit a limited tuning range (ca. 40 nm) due to material properties of silicon, and have difficulty maintaining a repeatable filter function during repeated tuning over long periods of time.
An anomalous technology, described in U.S. Pat. No. 5,068,861 entitled Etalon Apparatus, depicts a pair of mirrors separated by a compressible body portion (elastic spacer member) actuated with a helical compression spring via the rotation of a load adjustment knob. In theory, the helical compression spring could be controlled to squeeze the compressible body portion to select a desired wavelength for static operation. Clearly the manual operation of the adjustment knob and mechanical actuation are not suitable for scanning applications required by the telecommunications industry.
Thus, existing tunable Fabry-Perot Interferometers cannot provide a cost effective solution with the required Finesse and tuning range.
In view of the above limitations, the present invention provides a cost-effective broadband tunable Fabry-Perot interferometer.
This is accomplished with a pair of mirrored surfaces separated by an initial optical path length corresponding to the desired free spectral range. One of the mirrors is fixed while the other moves against the restoring force exerted by a compliant support. Tunability is afforded by creating field lines that exert a force by, for example, electrostatic or magnetic means that deforms the compliant support. When the force is removed, the energy stored in the compliant support restores the mirror to the initial separation.
In accordance with the present invention, the compliant support is formed of an entropic, rather than an enthalpic material, with a variety of geometries including compression, tension, sheer and diaphragm. Entropic materials afford four key advantages over enthalpic materials pertaining to device response and positional/angular stability.
(1) Entropic materials (e.g. long chain homopolymers, block copolymers, elastomers, aerogels etc.) exhibit an entropic plateau region (characterized by an elastic modulus that is ca. 5 MPa or less, and is independent of frequency and strain level over a wide range of frequencies and strain levels. Enthalpic materials have an elastic modulus that is ca. 1 GPa or more, and is independent of frequency only for very small strain levels.
(2) Entropic materials have a much higher elastic limit (more than ca. 100% strain vs. less than ca. 1% strain for enthalpic materials) and thus avoid plastic deformation during actuation. This greatly enhances the achievable tuning range.
(3) Entropic materials are incompressiblexcx9cthe energy cost for volume deformation is nearly infinite, when compared to the energy cost for linear and shear deformation. This compares with enthalpic materials wherein the energy cost for volume and linear deformations are comparable. When angular misalignment requires volume deformation in the support layer, and tuning requires shear or linear deformation only, then the energy cost for angular misalignment becomes much higher than the energy cost for tuning with an entropic support layer material, thus the device becomes more stable. For enthalpic support layers, the two energy costs remain comparable, thereby contributing to device instability.
(4) Entropic materials display a normal stress behavior: when they are shear deformed, they exert a so called normal stress perpendicular to the direction of shearing, in addition to the shear stress directly resulting from the shear strain. This behavior can be used to further enhance stability with specific compliant support geometries. Enthalpic materials do not display this normal stress behavior, and thus cannot be designed for enhanced stability.